Embedded Linux Experts

12 How to Get Your Change Into the Linux Kernel
3 or
4 Care And Operation Of Your Linus Torvalds
5678 For a person or company who wishes to submit a change to the Linux
9 kernel, the process can sometimes be daunting if you're not familiar
10 with "the system." This text is a collection of suggestions which
11 can greatly increase the chances of your change being accepted.
1213 This document contains a large number of suggestions in a relatively terse
14 format. For detailed information on how the kernel development process
15 works, see Documentation/development-process. Also, read
16 Documentation/SubmitChecklist for a list of items to check before
17 submitting code. If you are submitting a driver, also read
18 Documentation/SubmittingDrivers; for device tree binding patches, read
19 Documentation/devicetree/bindings/submitting-patches.txt.
2021 Many of these steps describe the default behavior of the git version
22 control system; if you use git to prepare your patches, you'll find much
23 of the mechanical work done for you, though you'll still need to prepare
24 and document a sensible set of patches. In general, use of git will make
25 your life as a kernel developer easier.
2627 --------------------------------------------
28 SECTION 1 - CREATING AND SENDING YOUR CHANGE
29 --------------------------------------------
303132 0) Obtain a current source tree
33 -------------------------------
3435 If you do not have a repository with the current kernel source handy, use
36 git to obtain one. You'll want to start with the mainline repository,
37 which can be grabbed with:
3839 git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
4041 Note, however, that you may not want to develop against the mainline tree
42 directly. Most subsystem maintainers run their own trees and want to see
43 patches prepared against those trees. See the "T:" entry for the subsystem
44 in the MAINTAINERS file to find that tree, or simply ask the maintainer if
45 the tree is not listed there.
4647 It is still possible to download kernel releases via tarballs (as described
48 in the next section), but that is the hard way to do kernel development.
4950 1) "diff -up"
51 ------------
5253 If you must generate your patches by hand, use "diff -up" or "diff -uprN"
54 to create patches. Git generates patches in this form by default; if
55 you're using git, you can skip this section entirely.
5657 All changes to the Linux kernel occur in the form of patches, as
58 generated by diff(1). When creating your patch, make sure to create it
59 in "unified diff" format, as supplied by the '-u' argument to diff(1).
60 Also, please use the '-p' argument which shows which C function each
61 change is in - that makes the resultant diff a lot easier to read.
62 Patches should be based in the root kernel source directory,
63 not in any lower subdirectory.
6465 To create a patch for a single file, it is often sufficient to do:
6667 SRCTREE= linux
68 MYFILE= drivers/net/mydriver.c
6970 cd $SRCTREE
71 cp $MYFILE $MYFILE.orig
72 vi $MYFILE # make your change
73 cd ..
74 diff -up $SRCTREE/$MYFILE{.orig,} > /tmp/patch
7576 To create a patch for multiple files, you should unpack a "vanilla",
77 or unmodified kernel source tree, and generate a diff against your
78 own source tree. For example:
7980 MYSRC= /devel/linux
8182 tar xvfz linux-3.19.tar.gz
83 mv linux-3.19 linux-3.19-vanilla
84 diff -uprN -X linux-3.19-vanilla/Documentation/dontdiff \
85 linux-3.19-vanilla $MYSRC > /tmp/patch
8687 "dontdiff" is a list of files which are generated by the kernel during
88 the build process, and should be ignored in any diff(1)-generated
89 patch.
9091 Make sure your patch does not include any extra files which do not
92 belong in a patch submission. Make sure to review your patch -after-
93 generating it with diff(1), to ensure accuracy.
9495 If your changes produce a lot of deltas, you need to split them into
96 individual patches which modify things in logical stages; see section
97 #3. This will facilitate review by other kernel developers,
98 very important if you want your patch accepted.
99100 If you're using git, "git rebase -i" can help you with this process. If
101 you're not using git, quilt <http://savannah.nongnu.org/projects/quilt>102 is another popular alternative.
103104105106 2) Describe your changes.
107 -------------------------
108109 Describe your problem. Whether your patch is a one-line bug fix or
110 5000 lines of a new feature, there must be an underlying problem that
111 motivated you to do this work. Convince the reviewer that there is a
112 problem worth fixing and that it makes sense for them to read past the
113 first paragraph.
114115 Describe user-visible impact. Straight up crashes and lockups are
116 pretty convincing, but not all bugs are that blatant. Even if the
117 problem was spotted during code review, describe the impact you think
118 it can have on users. Keep in mind that the majority of Linux
119 installations run kernels from secondary stable trees or
120 vendor/product-specific trees that cherry-pick only specific patches
121 from upstream, so include anything that could help route your change
122 downstream: provoking circumstances, excerpts from dmesg, crash
123 descriptions, performance regressions, latency spikes, lockups, etc.
124125 Quantify optimizations and trade-offs. If you claim improvements in
126 performance, memory consumption, stack footprint, or binary size,
127 include numbers that back them up. But also describe non-obvious
128 costs. Optimizations usually aren't free but trade-offs between CPU,
129 memory, and readability; or, when it comes to heuristics, between
130 different workloads. Describe the expected downsides of your
131 optimization so that the reviewer can weigh costs against benefits.
132133 Once the problem is established, describe what you are actually doing
134 about it in technical detail. It's important to describe the change
135 in plain English for the reviewer to verify that the code is behaving
136 as you intend it to.
137138 The maintainer will thank you if you write your patch description in a
139 form which can be easily pulled into Linux's source code management
140 system, git, as a "commit log". See #15, below.
141142 Solve only one problem per patch. If your description starts to get
143 long, that's a sign that you probably need to split up your patch.
144 See #3, next.
145146 When you submit or resubmit a patch or patch series, include the
147 complete patch description and justification for it. Don't just
148 say that this is version N of the patch (series). Don't expect the
149 subsystem maintainer to refer back to earlier patch versions or referenced
150 URLs to find the patch description and put that into the patch.
151 I.e., the patch (series) and its description should be self-contained.
152 This benefits both the maintainers and reviewers. Some reviewers
153 probably didn't even receive earlier versions of the patch.
154155 Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
156 instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
157 to do frotz", as if you are giving orders to the codebase to change
158 its behaviour.
159160 If the patch fixes a logged bug entry, refer to that bug entry by
161 number and URL. If the patch follows from a mailing list discussion,
162 give a URL to the mailing list archive; use the https://lkml.kernel.org/
163 redirector with a Message-Id, to ensure that the links cannot become
164 stale.
165166 However, try to make your explanation understandable without external
167 resources. In addition to giving a URL to a mailing list archive or
168 bug, summarize the relevant points of the discussion that led to the
169 patch as submitted.
170171 If you want to refer to a specific commit, don't just refer to the
172 SHA-1 ID of the commit. Please also include the oneline summary of
173 the commit, to make it easier for reviewers to know what it is about.
174 Example:
175176 Commit e21d2170f36602ae2708 ("video: remove unnecessary
177 platform_set_drvdata()") removed the unnecessary
178 platform_set_drvdata(), but left the variable "dev" unused,
179 delete it.
180181 You should also be sure to use at least the first twelve characters of the
182 SHA-1 ID. The kernel repository holds a *lot* of objects, making
183 collisions with shorter IDs a real possibility. Bear in mind that, even if
184 there is no collision with your six-character ID now, that condition may
185 change five years from now.
186187 If your patch fixes a bug in a specific commit, e.g. you found an issue using
188 git-bisect, please use the 'Fixes:' tag with the first 12 characters of the
189 SHA-1 ID, and the one line summary. For example:
190191 Fixes: e21d2170f366 ("video: remove unnecessary platform_set_drvdata()")
192193 The following git-config settings can be used to add a pretty format for
194 outputting the above style in the git log or git show commands
195196 [core]
197 abbrev = 12
198 [pretty]
199 fixes = Fixes: %h (\"%s\")
200201 3) Separate your changes.
202 -------------------------
203204 Separate each _logical change_ into a separate patch.
205206 For example, if your changes include both bug fixes and performance
207 enhancements for a single driver, separate those changes into two
208 or more patches. If your changes include an API update, and a new
209 driver which uses that new API, separate those into two patches.
210211 On the other hand, if you make a single change to numerous files,
212 group those changes into a single patch. Thus a single logical change
213 is contained within a single patch.
214215 The point to remember is that each patch should make an easily understood
216 change that can be verified by reviewers. Each patch should be justifiable
217 on its own merits.
218219 If one patch depends on another patch in order for a change to be
220 complete, that is OK. Simply note "this patch depends on patch X"
221 in your patch description.
222223 When dividing your change into a series of patches, take special care to
224 ensure that the kernel builds and runs properly after each patch in the
225 series. Developers using "git bisect" to track down a problem can end up
226 splitting your patch series at any point; they will not thank you if you
227 introduce bugs in the middle.
228229 If you cannot condense your patch set into a smaller set of patches,
230 then only post say 15 or so at a time and wait for review and integration.
231232233234 4) Style-check your changes.
235 ----------------------------
236237 Check your patch for basic style violations, details of which can be
238 found in Documentation/CodingStyle. Failure to do so simply wastes
239 the reviewers time and will get your patch rejected, probably
240 without even being read.
241242 One significant exception is when moving code from one file to
243 another -- in this case you should not modify the moved code at all in
244 the same patch which moves it. This clearly delineates the act of
245 moving the code and your changes. This greatly aids review of the
246 actual differences and allows tools to better track the history of
247 the code itself.
248249 Check your patches with the patch style checker prior to submission
250 (scripts/checkpatch.pl). Note, though, that the style checker should be
251 viewed as a guide, not as a replacement for human judgment. If your code
252 looks better with a violation then its probably best left alone.
253254 The checker reports at three levels:
255 - ERROR: things that are very likely to be wrong
256 - WARNING: things requiring careful review
257 - CHECK: things requiring thought
258259 You should be able to justify all violations that remain in your
260 patch.
261262263 5) Select the recipients for your patch.
264 ----------------------------------------
265266 You should always copy the appropriate subsystem maintainer(s) on any patch
267 to code that they maintain; look through the MAINTAINERS file and the
268 source code revision history to see who those maintainers are. The
269 script scripts/get_maintainer.pl can be very useful at this step. If you
270 cannot find a maintainer for the subsystem you are working on, Andrew
271 Morton (akpm@linux-foundation.org) serves as a maintainer of last resort.
272273 You should also normally choose at least one mailing list to receive a copy
274 of your patch set. linux-kernel@vger.kernel.org functions as a list of
275 last resort, but the volume on that list has caused a number of developers
276 to tune it out. Look in the MAINTAINERS file for a subsystem-specific
277 list; your patch will probably get more attention there. Please do not
278 spam unrelated lists, though.
279280 Many kernel-related lists are hosted on vger.kernel.org; you can find a
281 list of them at http://vger.kernel.org/vger-lists.html. There are
282 kernel-related lists hosted elsewhere as well, though.
283284 Do not send more than 15 patches at once to the vger mailing lists!!!
285286 Linus Torvalds is the final arbiter of all changes accepted into the
287 Linux kernel. His e-mail address is <torvalds@linux-foundation.org>.
288 He gets a lot of e-mail, and, at this point, very few patches go through
289 Linus directly, so typically you should do your best to -avoid-
290 sending him e-mail.
291292 If you have a patch that fixes an exploitable security bug, send that patch
293 to security@kernel.org. For severe bugs, a short embargo may be considered
294 to allow distributors to get the patch out to users; in such cases,
295 obviously, the patch should not be sent to any public lists.
296297 Patches that fix a severe bug in a released kernel should be directed
298 toward the stable maintainers by putting a line like this:
299300 Cc: stable@vger.kernel.org
301302 into the sign-off area of your patch (note, NOT an email recipient). You
303 should also read Documentation/stable_kernel_rules.txt in addition to this
304 file.
305306 Note, however, that some subsystem maintainers want to come to their own
307 conclusions on which patches should go to the stable trees. The networking
308 maintainer, in particular, would rather not see individual developers
309 adding lines like the above to their patches.
310311 If changes affect userland-kernel interfaces, please send the MAN-PAGES
312 maintainer (as listed in the MAINTAINERS file) a man-pages patch, or at
313 least a notification of the change, so that some information makes its way
314 into the manual pages. User-space API changes should also be copied to
315 linux-api@vger.kernel.org.
316317 For small patches you may want to CC the Trivial Patch Monkey
318 trivial@kernel.org which collects "trivial" patches. Have a look
319 into the MAINTAINERS file for its current manager.
320 Trivial patches must qualify for one of the following rules:
321 Spelling fixes in documentation
322 Spelling fixes for errors which could break grep(1)
323 Warning fixes (cluttering with useless warnings is bad)
324 Compilation fixes (only if they are actually correct)
325 Runtime fixes (only if they actually fix things)
326 Removing use of deprecated functions/macros
327 Contact detail and documentation fixes
328 Non-portable code replaced by portable code (even in arch-specific,
329 since people copy, as long as it's trivial)
330 Any fix by the author/maintainer of the file (ie. patch monkey
331 in re-transmission mode)
332333334335 6) No MIME, no links, no compression, no attachments. Just plain text.
336 -----------------------------------------------------------------------
337338 Linus and other kernel developers need to be able to read and comment
339 on the changes you are submitting. It is important for a kernel
340 developer to be able to "quote" your changes, using standard e-mail
341 tools, so that they may comment on specific portions of your code.
342343 For this reason, all patches should be submitted by e-mail "inline".
344 WARNING: Be wary of your editor's word-wrap corrupting your patch,
345 if you choose to cut-n-paste your patch.
346347 Do not attach the patch as a MIME attachment, compressed or not.
348 Many popular e-mail applications will not always transmit a MIME
349 attachment as plain text, making it impossible to comment on your
350 code. A MIME attachment also takes Linus a bit more time to process,
351 decreasing the likelihood of your MIME-attached change being accepted.
352353 Exception: If your mailer is mangling patches then someone may ask
354 you to re-send them using MIME.
355356 See Documentation/email-clients.txt for hints about configuring
357 your e-mail client so that it sends your patches untouched.
358359 7) E-mail size.
360 ---------------
361362 Large changes are not appropriate for mailing lists, and some
363 maintainers. If your patch, uncompressed, exceeds 300 kB in size,
364 it is preferred that you store your patch on an Internet-accessible
365 server, and provide instead a URL (link) pointing to your patch. But note
366 that if your patch exceeds 300 kB, it almost certainly needs to be broken up
367 anyway.
368369 8) Respond to review comments.
370 ------------------------------
371372 Your patch will almost certainly get comments from reviewers on ways in
373 which the patch can be improved. You must respond to those comments;
374 ignoring reviewers is a good way to get ignored in return. Review comments
375 or questions that do not lead to a code change should almost certainly
376 bring about a comment or changelog entry so that the next reviewer better
377 understands what is going on.
378379 Be sure to tell the reviewers what changes you are making and to thank them
380 for their time. Code review is a tiring and time-consuming process, and
381 reviewers sometimes get grumpy. Even in that case, though, respond
382 politely and address the problems they have pointed out.
383384385 9) Don't get discouraged - or impatient.
386 ----------------------------------------
387388 After you have submitted your change, be patient and wait. Reviewers are
389 busy people and may not get to your patch right away.
390391 Once upon a time, patches used to disappear into the void without comment,
392 but the development process works more smoothly than that now. You should
393 receive comments within a week or so; if that does not happen, make sure
394 that you have sent your patches to the right place. Wait for a minimum of
395 one week before resubmitting or pinging reviewers - possibly longer during
396 busy times like merge windows.
397398399 10) Include PATCH in the subject
400 --------------------------------
401402 Due to high e-mail traffic to Linus, and to linux-kernel, it is common
403 convention to prefix your subject line with [PATCH]. This lets Linus
404 and other kernel developers more easily distinguish patches from other
405 e-mail discussions.
406407408409 11) Sign your work
410 ------------------
411412 To improve tracking of who did what, especially with patches that can
413 percolate to their final resting place in the kernel through several
414 layers of maintainers, we've introduced a "sign-off" procedure on
415 patches that are being emailed around.
416417 The sign-off is a simple line at the end of the explanation for the
418 patch, which certifies that you wrote it or otherwise have the right to
419 pass it on as an open-source patch. The rules are pretty simple: if you
420 can certify the below:
421422 Developer's Certificate of Origin 1.1
423424 By making a contribution to this project, I certify that:
425426 (a) The contribution was created in whole or in part by me and I
427 have the right to submit it under the open source license
428 indicated in the file; or
429430 (b) The contribution is based upon previous work that, to the best
431 of my knowledge, is covered under an appropriate open source
432 license and I have the right under that license to submit that
433 work with modifications, whether created in whole or in part
434 by me, under the same open source license (unless I am
435 permitted to submit under a different license), as indicated
436 in the file; or
437438 (c) The contribution was provided directly to me by some other
439 person who certified (a), (b) or (c) and I have not modified
440 it.
441442 (d) I understand and agree that this project and the contribution
443 are public and that a record of the contribution (including all
444 personal information I submit with it, including my sign-off) is
445 maintained indefinitely and may be redistributed consistent with
446 this project or the open source license(s) involved.
447448 then you just add a line saying
449450 Signed-off-by: Random J Developer <random@developer.example.org>451452 using your real name (sorry, no pseudonyms or anonymous contributions.)
453454 Some people also put extra tags at the end. They'll just be ignored for
455 now, but you can do this to mark internal company procedures or just
456 point out some special detail about the sign-off.
457458 If you are a subsystem or branch maintainer, sometimes you need to slightly
459 modify patches you receive in order to merge them, because the code is not
460 exactly the same in your tree and the submitters'. If you stick strictly to
461 rule (c), you should ask the submitter to rediff, but this is a totally
462 counter-productive waste of time and energy. Rule (b) allows you to adjust
463 the code, but then it is very impolite to change one submitter's code and
464 make him endorse your bugs. To solve this problem, it is recommended that
465 you add a line between the last Signed-off-by header and yours, indicating
466 the nature of your changes. While there is nothing mandatory about this, it
467 seems like prepending the description with your mail and/or name, all
468 enclosed in square brackets, is noticeable enough to make it obvious that
469 you are responsible for last-minute changes. Example :
470471 Signed-off-by: Random J Developer <random@developer.example.org>472 [lucky@maintainer.example.org: struct foo moved from foo.c to foo.h]
473 Signed-off-by: Lucky K Maintainer <lucky@maintainer.example.org>474475 This practice is particularly helpful if you maintain a stable branch and
476 want at the same time to credit the author, track changes, merge the fix,
477 and protect the submitter from complaints. Note that under no circumstances
478 can you change the author's identity (the From header), as it is the one
479 which appears in the changelog.
480481 Special note to back-porters: It seems to be a common and useful practice
482 to insert an indication of the origin of a patch at the top of the commit
483 message (just after the subject line) to facilitate tracking. For instance,
484 here's what we see in a 3.x-stable release:
485486 Date: Tue Oct 7 07:26:38 2014 -0400
487488 libata: Un-break ATA blacklist
489490 commit 1c40279960bcd7d52dbdf1d466b20d24b99176c8 upstream.
491492 And here's what might appear in an older kernel once a patch is backported:
493494 Date: Tue May 13 22:12:27 2008 +0200
495496 wireless, airo: waitbusy() won't delay
497498 [backport of 2.6 commit b7acbdfbd1f277c1eb23f344f899cfa4cd0bf36a]
499500 Whatever the format, this information provides a valuable help to people
501 tracking your trees, and to people trying to troubleshoot bugs in your
502 tree.
503504505 12) When to use Acked-by: and Cc:
506 ---------------------------------
507508 The Signed-off-by: tag indicates that the signer was involved in the
509 development of the patch, or that he/she was in the patch's delivery path.
510511 If a person was not directly involved in the preparation or handling of a
512 patch but wishes to signify and record their approval of it then they can
513 ask to have an Acked-by: line added to the patch's changelog.
514515 Acked-by: is often used by the maintainer of the affected code when that
516 maintainer neither contributed to nor forwarded the patch.
517518 Acked-by: is not as formal as Signed-off-by:. It is a record that the acker
519 has at least reviewed the patch and has indicated acceptance. Hence patch
520 mergers will sometimes manually convert an acker's "yep, looks good to me"
521 into an Acked-by: (but note that it is usually better to ask for an
522 explicit ack).
523524 Acked-by: does not necessarily indicate acknowledgement of the entire patch.
525 For example, if a patch affects multiple subsystems and has an Acked-by: from
526 one subsystem maintainer then this usually indicates acknowledgement of just
527 the part which affects that maintainer's code. Judgement should be used here.
528 When in doubt people should refer to the original discussion in the mailing
529 list archives.
530531 If a person has had the opportunity to comment on a patch, but has not
532 provided such comments, you may optionally add a "Cc:" tag to the patch.
533 This is the only tag which might be added without an explicit action by the
534 person it names - but it should indicate that this person was copied on the
535 patch. This tag documents that potentially interested parties
536 have been included in the discussion.
537538539 13) Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
540 --------------------------------------------------------------------------
541542 The Reported-by tag gives credit to people who find bugs and report them and it
543 hopefully inspires them to help us again in the future. Please note that if
544 the bug was reported in private, then ask for permission first before using the
545 Reported-by tag.
546547 A Tested-by: tag indicates that the patch has been successfully tested (in
548 some environment) by the person named. This tag informs maintainers that
549 some testing has been performed, provides a means to locate testers for
550 future patches, and ensures credit for the testers.
551552 Reviewed-by:, instead, indicates that the patch has been reviewed and found
553 acceptable according to the Reviewer's Statement:
554555 Reviewer's statement of oversight
556557 By offering my Reviewed-by: tag, I state that:
558559 (a) I have carried out a technical review of this patch to
560 evaluate its appropriateness and readiness for inclusion into
561 the mainline kernel.
562563 (b) Any problems, concerns, or questions relating to the patch
564 have been communicated back to the submitter. I am satisfied
565 with the submitter's response to my comments.
566567 (c) While there may be things that could be improved with this
568 submission, I believe that it is, at this time, (1) a
569 worthwhile modification to the kernel, and (2) free of known
570 issues which would argue against its inclusion.
571572 (d) While I have reviewed the patch and believe it to be sound, I
573 do not (unless explicitly stated elsewhere) make any
574 warranties or guarantees that it will achieve its stated
575 purpose or function properly in any given situation.
576577 A Reviewed-by tag is a statement of opinion that the patch is an
578 appropriate modification of the kernel without any remaining serious
579 technical issues. Any interested reviewer (who has done the work) can
580 offer a Reviewed-by tag for a patch. This tag serves to give credit to
581 reviewers and to inform maintainers of the degree of review which has been
582 done on the patch. Reviewed-by: tags, when supplied by reviewers known to
583 understand the subject area and to perform thorough reviews, will normally
584 increase the likelihood of your patch getting into the kernel.
585586 A Suggested-by: tag indicates that the patch idea is suggested by the person
587 named and ensures credit to the person for the idea. Please note that this
588 tag should not be added without the reporter's permission, especially if the
589 idea was not posted in a public forum. That said, if we diligently credit our
590 idea reporters, they will, hopefully, be inspired to help us again in the
591 future.
592593 A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
594 is used to make it easy to determine where a bug originated, which can help
595 review a bug fix. This tag also assists the stable kernel team in determining
596 which stable kernel versions should receive your fix. This is the preferred
597 method for indicating a bug fixed by the patch. See #2 above for more details.
598599600 14) The canonical patch format
601 ------------------------------
602603 This section describes how the patch itself should be formatted. Note
604 that, if you have your patches stored in a git repository, proper patch
605 formatting can be had with "git format-patch". The tools cannot create
606 the necessary text, though, so read the instructions below anyway.
607608 The canonical patch subject line is:
609610 Subject: [PATCH 001/123] subsystem: summary phrase
611612 The canonical patch message body contains the following:
613614 - A "from" line specifying the patch author (only needed if the person
615 sending the patch is not the author).
616617 - An empty line.
618619 - The body of the explanation, line wrapped at 75 columns, which will
620 be copied to the permanent changelog to describe this patch.
621622 - The "Signed-off-by:" lines, described above, which will
623 also go in the changelog.
624625 - A marker line containing simply "---".
626627 - Any additional comments not suitable for the changelog.
628629 - The actual patch (diff output).
630631 The Subject line format makes it very easy to sort the emails
632 alphabetically by subject line - pretty much any email reader will
633 support that - since because the sequence number is zero-padded,
634 the numerical and alphabetic sort is the same.
635636 The "subsystem" in the email's Subject should identify which
637 area or subsystem of the kernel is being patched.
638639 The "summary phrase" in the email's Subject should concisely
640 describe the patch which that email contains. The "summary
641 phrase" should not be a filename. Do not use the same "summary
642 phrase" for every patch in a whole patch series (where a "patch
643 series" is an ordered sequence of multiple, related patches).
644645 Bear in mind that the "summary phrase" of your email becomes a
646 globally-unique identifier for that patch. It propagates all the way
647 into the git changelog. The "summary phrase" may later be used in
648 developer discussions which refer to the patch. People will want to
649 google for the "summary phrase" to read discussion regarding that
650 patch. It will also be the only thing that people may quickly see
651 when, two or three months later, they are going through perhaps
652 thousands of patches using tools such as "gitk" or "git log
653 --oneline".
654655 For these reasons, the "summary" must be no more than 70-75
656 characters, and it must describe both what the patch changes, as well
657 as why the patch might be necessary. It is challenging to be both
658 succinct and descriptive, but that is what a well-written summary
659 should do.
660661 The "summary phrase" may be prefixed by tags enclosed in square
662 brackets: "Subject: [PATCH <tag>...] <summary phrase>". The tags are
663 not considered part of the summary phrase, but describe how the patch
664 should be treated. Common tags might include a version descriptor if
665 the multiple versions of the patch have been sent out in response to
666 comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
667 comments. If there are four patches in a patch series the individual
668 patches may be numbered like this: 1/4, 2/4, 3/4, 4/4. This assures
669 that developers understand the order in which the patches should be
670 applied and that they have reviewed or applied all of the patches in
671 the patch series.
672673 A couple of example Subjects:
674675 Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
676 Subject: [PATCH v2 01/27] x86: fix eflags tracking
677678 The "from" line must be the very first line in the message body,
679 and has the form:
680681 From: Original Author <author@example.com>682683 The "from" line specifies who will be credited as the author of the
684 patch in the permanent changelog. If the "from" line is missing,
685 then the "From:" line from the email header will be used to determine
686 the patch author in the changelog.
687688 The explanation body will be committed to the permanent source
689 changelog, so should make sense to a competent reader who has long
690 since forgotten the immediate details of the discussion that might
691 have led to this patch. Including symptoms of the failure which the
692 patch addresses (kernel log messages, oops messages, etc.) is
693 especially useful for people who might be searching the commit logs
694 looking for the applicable patch. If a patch fixes a compile failure,
695 it may not be necessary to include _all_ of the compile failures; just
696 enough that it is likely that someone searching for the patch can find
697 it. As in the "summary phrase", it is important to be both succinct as
698 well as descriptive.
699700 The "---" marker line serves the essential purpose of marking for patch
701 handling tools where the changelog message ends.
702703 One good use for the additional comments after the "---" marker is for
704 a diffstat, to show what files have changed, and the number of
705 inserted and deleted lines per file. A diffstat is especially useful
706 on bigger patches. Other comments relevant only to the moment or the
707 maintainer, not suitable for the permanent changelog, should also go
708 here. A good example of such comments might be "patch changelogs"
709 which describe what has changed between the v1 and v2 version of the
710 patch.
711712 If you are going to include a diffstat after the "---" marker, please
713 use diffstat options "-p 1 -w 70" so that filenames are listed from
714 the top of the kernel source tree and don't use too much horizontal
715 space (easily fit in 80 columns, maybe with some indentation). (git
716 generates appropriate diffstats by default.)
717718 See more details on the proper patch format in the following
719 references.
720721 15) Explicit In-Reply-To headers
722 --------------------------------
723724 It can be helpful to manually add In-Reply-To: headers to a patch
725 (e.g., when using "git send-email") to associate the patch with
726 previous relevant discussion, e.g. to link a bug fix to the email with
727 the bug report. However, for a multi-patch series, it is generally
728 best to avoid using In-Reply-To: to link to older versions of the
729 series. This way multiple versions of the patch don't become an
730 unmanageable forest of references in email clients. If a link is
731 helpful, you can use the https://lkml.kernel.org/ redirector (e.g., in
732 the cover email text) to link to an earlier version of the patch series.
733734735 16) Sending "git pull" requests
736 -------------------------------
737738 If you have a series of patches, it may be most convenient to have the
739 maintainer pull them directly into the subsystem repository with a
740 "git pull" operation. Note, however, that pulling patches from a developer
741 requires a higher degree of trust than taking patches from a mailing list.
742 As a result, many subsystem maintainers are reluctant to take pull
743 requests, especially from new, unknown developers. If in doubt you can use
744 the pull request as the cover letter for a normal posting of the patch
745 series, giving the maintainer the option of using either.
746747 A pull request should have [GIT] or [PULL] in the subject line. The
748 request itself should include the repository name and the branch of
749 interest on a single line; it should look something like:
750751 Please pull from
752753 git://jdelvare.pck.nerim.net/jdelvare-2.6 i2c-for-linus
754755 to get these changes:
756757 A pull request should also include an overall message saying what will be
758 included in the request, a "git shortlog" listing of the patches
759 themselves, and a diffstat showing the overall effect of the patch series.
760 The easiest way to get all this information together is, of course, to let
761 git do it for you with the "git request-pull" command.
762763 Some maintainers (including Linus) want to see pull requests from signed
764 commits; that increases their confidence that the request actually came
765 from you. Linus, in particular, will not pull from public hosting sites
766 like GitHub in the absence of a signed tag.
767768 The first step toward creating such tags is to make a GNUPG key and get it
769 signed by one or more core kernel developers. This step can be hard for
770 new developers, but there is no way around it. Attending conferences can
771 be a good way to find developers who can sign your key.
772773 Once you have prepared a patch series in git that you wish to have somebody
774 pull, create a signed tag with "git tag -s". This will create a new tag
775 identifying the last commit in the series and containing a signature
776 created with your private key. You will also have the opportunity to add a
777 changelog-style message to the tag; this is an ideal place to describe the
778 effects of the pull request as a whole.
779780 If the tree the maintainer will be pulling from is not the repository you
781 are working from, don't forget to push the signed tag explicitly to the
782 public tree.
783784 When generating your pull request, use the signed tag as the target. A
785 command like this will do the trick:
786787 git request-pull master git://my.public.tree/linux.git my-signed-tag
788789790 ----------------------
791 SECTION 2 - REFERENCES
792 ----------------------
793794 Andrew Morton, "The perfect patch" (tpp).
795 <http://www.ozlabs.org/~akpm/stuff/tpp.txt>796797 Jeff Garzik, "Linux kernel patch submission format".
798 <http://linux.yyz.us/patch-format.html>799800 Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".
801 <http://www.kroah.com/log/linux/maintainer.html>802 <http://www.kroah.com/log/linux/maintainer-02.html>803 <http://www.kroah.com/log/linux/maintainer-03.html>804 <http://www.kroah.com/log/linux/maintainer-04.html>805 <http://www.kroah.com/log/linux/maintainer-05.html>806 <http://www.kroah.com/log/linux/maintainer-06.html>807808 NO!!!! No more huge patch bombs to linux-kernel@vger.kernel.org people!
809 <https://lkml.org/lkml/2005/7/11/336>
810811 Kernel Documentation/CodingStyle:
812 <Documentation/CodingStyle>
813814 Linus Torvalds's mail on the canonical patch format:
815 <http://lkml.org/lkml/2005/4/7/183>816817 Andi Kleen, "On submitting kernel patches"
818 Some strategies to get difficult or controversial changes in.
819http://halobates.de/on-submitting-patches.pdf820821 --

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